Google Financehttps://finance.google.com/finance
Google Finance is a quick source for data on stock prices and market indices. With a bit of trickery, you can use it as a source to build datasets of stock prices.

Federal Reserve Board: Data Releaseshttp://www.federalreserve.gov/econresdata/statisticsdata.htm
The Board of Governors of the Federal Reserve system publishes a series of data releases on monetary and financial indicators on topics such as household finances, interest rates, exchange rates, industrial production, and monetary aggregates. Data from the various releases can be queried and downloaded via the Data Download Program.

Federal Reserve Economic Data (FRED II)http://research.stlouisfed.org/fred2/
FRED II includes time-series data for variables such as GDP, interest rates, exchange rates, consumer prices, and banking. There also also add-ins available to access and use FRED data in Excel, Stata, Stata (again), and R. Most of the data are from the 1950's onwards, though some series extend back prior to WWII.

Wharton Research Data Services (WRDS)https://wrds-web.wharton.upenn.edu/wrds/
WRDS is an excellent source for data on both company financials (via COMPUSTAT) and stock prices (via CRSP). The university's WRDS subscription also provides access to data for measures of market volatility, balance sheets of financial institutions, and models of asset performance. Access to the university's WRDS subscription requires registration to request an account.

Yahoo! Financehttps://finance.yahoo.com/
Yahool Finance is another quick source for financial data and is a useful way to get historical data on individual stocks.